Undo sync

Hello,

I just got an iPhone 4s and I love it (Its my first iPhone too).

Earlier today, I dragged and dropped all my music from my MacBook Pro onto the 'Music' folder in my iPhone. and,for some stupid reason while trying new stuff on my phone a while later, I hit the 'sync' option on the laptop with my iPhone menu open. Now,I lost all the music on my phone.

1.The music is still there on my iTunes library but I am unable to put it into my phone again.

How do I fix this? please let me know.

Thank you.